I signed up for the reading pledge and I have added some books in my shelf, however, after a few days they are deleted. Is there a time limit?

I wanted to know why I post some books on my shelf but after a few days they are taken off my shelf. It also says that my shelf has no books added. I don't know why this is happening. I am signed in my account and the books appear to be on the shelf. Is there a limit of time and then the books are removed from the shelf?